As wireless phone standards continue to evolve to 3G and beyond, for example, WCDMA (Wideband Code Division Multiple Access), HSUPA (High-Speed Uplink Packet Access), and eventually 3G LTE (Long-Term Evolution), the demand for non-constant envelope modulation is growing rapidly. There is an increasing need for modulation and power-control schemes that permit nonlinear power amplifiers to operate in the saturation (nonlinear) region, which provides higher power efficiency and longer battery lifetime.
In nonlinear devices, waveform quality typically increases with a more linear output response. However, some nonlinear devices operate more efficiently when the output response is nonlinear—for example, when a power amplifier approaches saturation. As a result, there is often a tradeoff between waveform quality and efficiency. For example, when a nonlinear device approaches saturation or starts to exhibit nonlinear qualities (which may improve efficiency), the waveform quality may be degraded and may not meet the specific requirements and standards. Alternatively, if nonlinear devices are set to operate in linear regions to meet quality standards or requirements, then power consumption and current drain may be degraded because the device is operating at a lower efficiency level.
Correction of the nonlinearity of the power amplifier becomes extremely challenging in the context of non-constant envelope modulation. Two primary approaches exist: open-loop LUT (lookup table) correction and closed-loop error based correction. Open-loop correction is relatively simple, but needs significant manufacturing calibration for collecting tables or calculating the coefficients, and suffers performance loss if the device nonlinearity varies once out of the manufacturing environment and the pre-collected tables or pre-calculated coefficients are no longer accurate enough. The error based closed-loop correction, either non-adaptive or adaptive, is a classical linear control loop scheme and fails at delivering accurate correction to severe nonlinearity, especially for a high gain loop with large delay.
A third, hybrid approach has been proposed for certain polar modulation schemes, particularly for use with GSM/EDGE. This hybrid polar modulation approach involves generating an LUT in real-time during a closed-loop calibration interval and then using the LUT to correct a signal during an open-loop operation interval. This approach is suitable for use with GSM/EDGE, but newer 3G modulation schemes, such as WCDMA, HSUPA, and LTE, introduce significant new challenges in synchronizing the signal data in the LUT. The hybrid polar modulation approach can maintain synchronization with timing misalignments of 20 ns (nanoseconds) or less, which is tolerable for purposes of GSM/EDGE. HSUPA, by contrast, is estimated to tolerate a maximum timing misalignment of approximately 2 ns—ten times smaller than the tolerable misalignment for GSM/EDGE. To achieve this high level of synchronization with the hybrid polar modulation approach would require a dedicated synchronization solution, including additional hardware and complexity.
Thus, there is a need for reliable and efficient systems and methods for efficient, highly-synchronized, linearly-corrected modulation in communication transmission systems. There is a further need for reliable and efficient systems and methods for synchronized, pre-distorted I/Q modulation for use with non-linear power amplifiers.